The Torah refers to the frogs in the singular. The Sages disagree with regard to the reason the Torah uses the singular form of the word. They also speak in praise of the frogs, which endangered their lives to punish the Egyptians.

“The frogs [hatzefarde’a, in the singular] arose and covered the land of Egypt” (Exodus 8:2). Rabbi Akiva says: It was one enormous frog that filled the entire land of Egypt. Rabbi Elazar ben Azarya said to him: Akiva, what are you doing involving yourself with Aggada? Cease making statements in these matters and go to study Nega’im and Oholot [complex tractates that discuss halakhot of ritual impurity]. It was one frog that whistled for others, and they came. The verse relates that “the Nile will swarm with frogs, and they will arise and come into your house…and into your ovens and into your kneading bowls” (7:28). When the Egyptian woman would knead the dough and ignite the oven, the frogs would enter the dough and eat the dough, and enter the oven and cool it, and would stick to the bread…. Hananya, Mishael, and Azarya drew an (Sanhedrin 67b; Shemot Rabba 10)